Sticking a battery on your home solar system seems like the simplest idea in the world. You could use it to ride through power outages, or run appliances on stored solar power when grid demand is peaking to avoid brownouts or earn cash back on your power bill. You could even charge your plug-in car with it.

But at what cost? So far, home battery makers have been unable to prove their systems can compete with the grid’s cheap and highly reliable power. Why pay thousands of dollars for backup batteries when the power only goes out twice a year?

The first bundled product offering will be ready by September of this year, the companies announced. Hanwha Group, Hanwha Solar’s parent company, also led a Series B funding round with an $8 million investment.

I talked to Baxter, Minn.-based Silent Power in 2009, when CEO Todd Headlee described a plan to give utilities control over lots of batteries connected to residential solar systems for grid balancing. Silent Power was piloting the concept at the time, Headlee said, though he remained silent about which battery and solar companies it might partner with to get to market.

More recently, Headlee told Greentech Media that the company owns its own proprietary inverter technology developed specifically for the Hanwha application, and is working with sealed AGM lead acid and lithium-ion battery technologies, depending on utility preference.

The AGM lead-acid configuration yields about five years of life and lithium-ion can get up to 20 years, but at a much higher initial cost, he said. It’s aimed at both residential and commercial markets, and promises peak-shifting -- say, from solar’s 3 p.m. to 4 p.m. peak to the 5 p.m. to 7 p.m. power demand peak when everyone comes home from work -- as well as backup power during outages.

The whole thing uses AMI, or smart meters, as the communication network to Silent Power’s devices, Headlee said. A company spokesman said Silent Power’s system integrates to utility AMI, cellular and internet demand response systems to allow access to power stored as needed.

But that blueprint also includes a battery. Without it, solar may be too unstable an energy source to incorporate into utility grids in a big way.

Silent Power has a solar inverter-battery rig it says can accomplish that balancing task. In fact, the Baxter, Minn.-based startup is involved with GE in at least a few smart grid projects seeking stimulus funding to install those systems across a large number of homes, CEO Todd Headlee said last week at the Clean Energy Venture Summit in Austin, Texas.

Silent Power has raised $4 million in angel financing, and is seeking to raise a $7 million series A round to "attack the utility space," with commercialization planned for 2011, Headlee said.

As for GE, "We've partnered with them on a couple of proposals," he said, though he wouldn't give additional details on the projects the two were planning (see Green Light post).

The problem with utility-scale energy storage has so far been its price. Beyond huge, capital-intensive and site-limited systems like pumped hydro and compressed air energy storage, batteries, flywheels, fuel cells and other distributed energy storage technologies are still too expensive for most utilities to justify (see "Grid Energy Storage: Big Market, Tough to Tackle").

Silent Energy says it can drastically undercut those costs by maxing out the federal tax rebate available for solar system installations -- and getting utilities to foot part of the remaining bill.

A Silent Power system should add about $5,000 to the cost of a typical solar installation, Headlee said. The 30-percent federal tax credit shaves that by 30 percent, leaving about $3,500 remaining -- or, as Silent Power likes to frame it, about $350 per kilowatt-hour in storage capacity. That's cheaper than commercially available battery storage systems today, though it's a target price for many.

Granted, the batteries won't last more than a few hours, he said. But that's enough for utilities to smooth the difference between the time that solar panels get their best output, in the mid-afternoon, and the slightly later peak demand times that are typical for utilities during hot summer afternoons as people get home from work and start turning stuff on, he said.

That demand response capacity could be worth something to utilities -- perhaps even a $3,500 rebate or incentive to homeowners to make Silent Power's system as cheap as a regular solar panel array, he suggested.

A solar-battery system would require inverter systems that can switch instantly between the two -- something Silent Power's system can do, Headlee said. About 200 homes with Silent Power's units should give a utility 1 megawatt of load-shifting capacity, he estimated.

Beyond that, batteries can help smooth the big swings in solar panel generation when clouds pass overhead, he said. Spread out to neighborhood scale, such drastic ups and downs in home power demand could spell trouble for an unprepared distribution grid.

The market is huge, Headlee said. American Electric Power, for one, says it will need 1 gigawatt of storage, or 2.6 percent of generation capacity, by 2020, he noted, and California may require 4 gigawatts of storage to reach its goal of getting a third of its power from renewable resources by 2020. About 20 gigawatts of new storage would represent a $10 billion market, he said (see Green Light post).

For now, the company plans to use lead-acid batteries with an estimated five- to seven-year lifespan, he said. But it's looking into lithium-ion and advanced lead-acid batteries as well, he said.

A few venture capitalists briefed on Silent Power's plans at last week's conference in Austin noted that the company might be a bit early to the market. Systems for allowing utilities to control large numbers of home-based batteries are in their infancy, though the millions of smart meters being deployed could help extend those networks, Headlee said.

In fact, smart grid startup GridPoint started out in 2003 with a similar business plan, then switched to software after finding the market too small, he said (see "GridPoint Gets $120M, Buys V2Green").

But then, utilities have been running demand response programs for years, with factories and other large power users turning down their use at peak demand times, Headlee said (see "Is Demand Response Doomed?")

And utilities expecting hundreds of megawatts of rooftop solar panels to be installed in their service territories may feel under the gun to put storage in place to make it an asset, rather than a grid-destabilizing liability, he said.

Interact with smart grid industry visionaries from North American utilities, innovative hardware and software vendors, and leading industry consortiums at The Networked Grid on November 4 in San Francisco.